nvme: make nvmf_register_transport require a create_ctrl callback

nvmf_create_ctrl() relys on the presence of a create_crtl callback in the
registered nvmf_transport_ops, so make nvmf_register_transport require one.

Update the available call-sites as well to reflect these changes.

Signed-off-by: Johannes Thumshirn <jthumshirn@suse.de>
Signed-off-by: Sagi Grimberg <sagi@grimberg.me>
Signed-off-by: Jens Axboe <axboe@fb.com>
diff --git a/drivers/nvme/host/fabrics.c b/drivers/nvme/host/fabrics.c
index 916d136..5b7386f 100644
--- a/drivers/nvme/host/fabrics.c
+++ b/drivers/nvme/host/fabrics.c
@@ -480,11 +480,16 @@ EXPORT_SYMBOL_GPL(nvmf_connect_io_queue);
  * being implemented to the common NVMe fabrics library. Part of
  * the overall init sequence of starting up a fabrics driver.
  */
-void nvmf_register_transport(struct nvmf_transport_ops *ops)
+int nvmf_register_transport(struct nvmf_transport_ops *ops)
 {
+	if (!ops->create_ctrl)
+		return -EINVAL;
+
 	mutex_lock(&nvmf_transports_mutex);
 	list_add_tail(&ops->entry, &nvmf_transports);
 	mutex_unlock(&nvmf_transports_mutex);
+
+	return 0;
 }
 EXPORT_SYMBOL_GPL(nvmf_register_transport);